1. Field of the Invention
This invention relates to dump tanks and has particular reference to emergency dump tanks for the retention of congealable materials.
The invention has further particular, but not necessarily exclusive, reference to dump tanks for use in the containment of solutions of cellulose in aqueous organic compounds--typically n-methyl morpholine n-oxide.
Cellulosic fibres have attractive absorbency and comfort characteristics and are widely used in the production of products such as garments and absorbency products. Certain cellulosic fibres occur in nature--such as cotton. Other cellulosic fibres are produced by the formation of a chemical compound of cellulose to produce a spinnable solution which is extruded or spun into a regeneration bath to form the fibres. Viscose rayon is an example of such a fibre.
More recently techniques have been developed for the production of cellulosic fibres by the dissolution of cellulose in an aqueous organic solvent for cellulose. Such fibres have been given the generic term "lyocell fibres".
A preferred solvent for cellulose is a tertiary amine oxide, in particular n-methyl morpholine n-oxide.
Although cellulose can form a stable solution in an aqueous n-methyl morpholine solvent, the solution can exotherm if the temperature of the solution rises above a predetermined level. Once under way, an exotherm comprises an uncontrollable chemical reaction characterised by the release of excessive quantities of energy in a very short period of time. Because the solution of cellulose in aqueous n-methyl morpholine n-oxide (NMMO) is relatively viscous--although thixotropic--one of the characteristics of an exotherm is the displacement of solution as a result of the formation of gases in the exotherm. Unstabilised solutions gradually degrade, in doing so they emit heat. This can cause a runaway reaction once the temperature of the solution reaches about 170.degree. C. Stabilised solutions using stabilisers such as propyl gallate reach runaway reactions temperatures at about 180.degree. C.
Proposals have been made, therefore, to provide safety relief devices to relieve pressure and material in the event of an exotherm. Such a device is described in copending application Ser. No. 08/066,532 filed on May 24, 1993 the contents of which are incorporated herein by way of reference, and which is based on the priority of UK Patent Application No. 9220407.2 originally filed in the United Kingdom on Sep. 28, 1992.
The present invention is concerned with an emergency dump tank for the retention of congealable material produced, in a particular example, by an exotherm occurring in a pipeline containing a solution of cellulose in an aqueous NMMO solution.
By congealable material as used herein is meant a material which increases significantly in viscosity or cooling or coming to rest, forming for example a solid or a rubbery mass of material or a material of too high a viscosity to flow or be pumped.